Bow Saw Geometry

[Josh Nelson]

Good evening all! I've found myself in need of a new bow saw for green wood. My large metal monstrosity has finally bit the dust (my son ran over it.... just... I don't even know how) but while the blade was salvageable, the frame has not. So for a project I've decided to build one myself. I've got the wood picked out and the tension considerations taken care of but I'm curious if there is a good geometric rule of thumb here for deciding the length of the arms. The depth of cut will be 12", the length of the blade is 36", so I'm curious if and equal length above the span (12") would be advantageous or whether it should be shorter (or longer). Most of the old style bow and buck saws I've seen are a bit shorter on the tops of the arms...

Any suggestions would be great!

[Kevin Hampshire]

Josh, you might try an image search on the type of saw that you're interested in. When you mention a metal frame and 36 inches, it sounds to me like it could also be called a buck saw. When I searched on bucksaw I saw quite a few different frames which might give you some idea.

[Lonnie Gallaher]

I have an old wood "buck" saw that I picked up at a garage sale some time back. The saw is not serviceable, it has spent too much time in the weather. The 1 1/2 blade is 25 inches between the arms. It has a depth of cut of just over 11 inches and it is 18 inches to the centerline of the turnbuckle tensioner to the tooth line. So the geometry is 2 to 1, considering the centerline of the beam.

[Patrick Chase]

Good evening all! I've found myself in need of a new bow saw for green wood. My large metal monstrosity has finally bit the dust (my son ran over it.... just... I don't even know how) but while the blade was salvageable, the frame has not. So for a project I've decided to build one myself. I've got the wood picked out and the tension considerations taken care of but I'm curious if there is a good geometric rule of thumb here for deciding the length of the arms. The depth of cut will be 12", the length of the blade is 36", so I'm curious if and equal length above the span (12") would be advantageous or whether it should be shorter (or longer). Most of the old style bow and buck saws I've seen are a bit shorter on the tops of the arms...

Any suggestions would be great!

It's a matter of mechanical advantage. The shorter the arms are above the crossbar the more you have to tension the top member (twine, rod, whatever) for any desired amount of blade tension. Specifically:

top_tension = blade_tension*arm_length_below_span/arm_length_above_span

And:

compression_on_span = blade_tension*(1 + arm_length_below/arm_length_above)

In other words, if you shorten the arms above the span then you have to make sure that the tensioner and the compression member in the middle can apply (in the case of the tensioner) or handle (in the case of the cross member) more force. It's a fairly straightforward tradeoff IMO.

This is exactly why I asked Allen Speers about blade tension in the Nobex Pro miter box+saw in the "what is the hand equivalent of a crosscut sled" thread. (http://www.sawmillcreek.org/showthread.php?243663-What-is-the-hand-equivalent-of-a-crosscut-sled&p=2562322#post2562322) The Nobex saw has almost 3X as much clearance below the cross member as above. What I failed to notice before I posted is just how beefy that cross member is - it can easily handle the additional load.

EDIT: In case it isn't obvious, each arm in a bowsaw is a lever, with the fulcrum of each such lever where it meets the middle compression member. One you start thinking about it in those terms the rest is just basic mechanics.

[Denys Sushko]

Hi? do you really need 12" capacity? you can twist the blade for 15-30° and stretcher will be on the right and not directly above the kerf

[lowell holmes]

This might be a place to start,

http://www.highlandwoodworking.com/bow-saw-frame-saws-blades.aspx

[Luke Dupont]

It's a matter of mechanical advantage. The shorter the arms are above the crossbar the more you have to tension the top member (twine, rod, whatever) for any desired amount of blade tension. Specifically:

top_tension = blade_tension*arm_length_below_span/arm_length_above_span

And:

compression_on_span = blade_tension*(1 + arm_length_below/arm_length_above)

In other words, if you shorten the arms above the span then you have to make sure that the tensioner and the compression member in the middle can apply (in the case of the tensioner) or handle (in the case of the cross member) more force. It's a fairly straightforward tradeoff IMO.

This is exactly why I asked Allen Speers about blade tension in the Nobex Pro miter box+saw in the "what is the hand equivalent of a crosscut sled" thread. (http://www.sawmillcreek.org/showthread.php?243663-What-is-the-hand-equivalent-of-a-crosscut-sled&p=2562322#post2562322) The Nobex saw has almost 3X as much clearance below the cross member as above. What I failed to notice before I posted is just how beefy that cross member is - it can easily handle the additional load.

EDIT: In case it isn't obvious, each arm in a bowsaw is a lever, with the fulcrum of each such lever where it meets the middle compression member. One you start thinking about it in those terms the rest is just basic mechanics.

You must be a programmer, Patrick!

But, basically, what Patrick said! I have had problems with bow saws that have too little leverage - you can go this route, but you'll need a very strong cord.

By the way, I suggest Bahco Drywood blades for green wood. They seem to be the best rated among the bushcraft crowd.

[Mike Holbrook]

Take a look at the bowsaws made by Glenn at WoodJoy Tools. There are a few distinctions in the design that I find important. The frames are extra sturdy and precisely made. The heavy waxed cord Glenn uses works better than a metal rod & nut, other thin lines...Glenn uses a winder on his bowsaw strings that is better engineered than anything else I have seen. There are two precisely made pieces. The winder will make half turns and make all the tension needed in less turns.

Glenn sells parts as well as fully made saws. At the price Glenn sells his saws for I just bought one.